1. Field of the Invention
This invention relates to electroexplosive devices and to static discharge elements for use in such devices. More particularly, this invention relates to a novel static discharge element for use in an electroexplosive device.
The term, "electroexplosive device" or (or EED) herein refers to any electrically initiated explosive or pyrotechnic device. Such devices include, for example, squibs, initiators, electric initiators, electric detonators, and electrically initiated matches.
2. Description of the Prior Art
Airbags have been suggested as a means for protecting passengers of automobiles and other vehicles from injury due to striking a part of the vehicle (such as the windshield or dash board) in the event of rapid deceleration, which may occur in the event of a crash. An advantage of the airbag over other passenger restraint devices, such as seat belts, is that the airbag is initiated automatically by rapid deceleration and does not require any action on the part of a passenger (such as fastening a seat belt).
The rapid action required for inflating an airbag is best provided by an EED. However, either static electricity, radio frequency (RF) waves or both, may be present in the vicinity of an automobile. Either one is capable of accidental initiation of an EED. U.S. Pat. No. 3,414,292 to Oldberg et al. shows an airbag initiated by an EED and having means located externally of the EED for preventing accidental initiation by radio frequency (RF) currents. Provision of means for preventing accidental RF initiation is essential in EED's used in automobiles.
An EED having both a ferrite plug located inside the casing for protection against RF discharge, and means (a resistor) for preventing accidental electrostatic discharge, is shown in U.S. Pat. No. 3,264,989 to Rucker.
Numerous patents illustrate EED's containing a static discharge element in the form of a semiconductive plug, or "static shunt mix", consisting of metal powder such as alumina dispersed in a nonconductive binder such as wax or polyethylene. Such EED's are shown for example in U.S. Pat. Nos. 2,658,451 to Horne, 2,802,421 to Horne et al., and 3,194,160 to Spillane et al. A semiconductive plug presents a conductive discharge path for high voltage discharges and a high resistance path for the low voltages normally used to fire EED's. Disadvantages of semiconductive mixes are twofold. First of all, dielectric strength and insulation resistance are relatively low and variable. The second disadvantage is that the static discharge mix is of paste consistency and must be introduced into the EED in precise amounts, which is difficult and expensive because of the small sizes of most EED's.
Another type of static shunt device is shown in U.S. Pat. No. 3,333,538 to Schnettler. This patent shows a thin nonconductive plastic sheet having a plurality of conductive hexagon-shaped areas, separated by spark gaps formed by the uncoated spaces between the hexagons. The hexagons are dimensioned so that one gap is always provided between each lead wire and the shell, and so that there is always at least one gap between the lead wires. The plastic sheet is pierced by the lead wires during assembly, which results in firm electrical contact between the lead wires and the conductive areas on the sheet. One disadvantage of the Schnettler structure is that the sheet must be oriented during assembly so that the rows of hexagons are parallel to the line connecting centers of the lead wires. Another disadvantage is there is some danger of bending the lead wires during assembly, because or clearance is provided between the leads and the sheet. Another disadvantage is that the leads must be straight at the time of assembly of the static shunt device. Also, the distance between lead wires must equal or exceed the distance from either lead wire to the casing.
Another type of static discharge device is illustrated in U.S. Pat. No. 3,789,762 to Petrick. This static discharge device comprises a tab of metallic foil which is connected to the metallic casing of the EED and which has a pair of points that are in proximity with the lead wires of the EED. This structure provides a pair of spark gaps from each of the lead wires to the metal foil. Proper operation of this device depends on precise control of spark gap distances, so that currents induced by static electricity will jump across the spark gaps from the leads to the metal foil. However, because of the small size of most EED's and the flexible nature of the metal foil, it is difficult to achieve uniform spark gaps. Either a slight departure from the desired or nominal spacing of the lead wires, or a slight bending of the points, may cause the spark gap distance to increase substantially and thereby reduce the protection offered by the device.
U.S. Pat. No. 4,061,088 to Ueda discloses an EED containing a nonlinear resistor element which prevent ignition in the event of a static discharge.
Although numerous static discharge devices are known in the art, none to date has all properties desired in a static discharge element, such as low cost and ease of assembly, high dielectric strength, and high degree of reliability.